The present invention relates to a process for the manufacture of through-hole plated electric printed-circuit boards, using a laminate which comprises at least one layer of an insulating material in the form of a board or a web and at least one electrically conductive metal layer and shows a plurality of plated-through holes.
It is known to manufacture circuit boards for printed circuits from a laminate which comprises an insulation board, for example, of a glass-fiber reinforced phenolic resin, and a thin copper layer bonded to it. In the process, the copper layer is covered with a stencil corresponding to the arrangement of the required conductive paths and the copper which has not been covered is removed by etching. In addition to this operating procedure which is called the subtractive method, there is the additive method, in which the surface of the insulation board is provided with noble-metal nuclei and covered in image configuration. Thereafter, the surface which is not covered is metallized in a chemical plating process and, if appropriate, an electroplating process. Moreover, a semiadditive method is customarily used, in which first a continuous, thin conductive copper layer is deposited, then the covering stencil is applied, the areas which lie open are reinforced by electroplating, the stencil is detached and the uncovered areas of the non-reinforced copper layer are etched away (cf. Herrmann, "Handbuch der Leiterplattentechnik" (Manual of Printed-Circuit Board Technique), publishers Eugen G. Leuze-Verlag, Saulgau, Federal Republic of Germany, 2nd edition, 1982).
Up to the present, the holes for through-plating are produced by drilling or punching the boards, corresponding to the required size and number of holes, i.e., holes are provided only in those places where they are needed. Then the walls of the holes are rendered conductive by a preliminary electroless deposition of metal.
These two process steps--drilling or punching of holes and through-plating--are performed according to the presently known and customary processes, in a relatively late stage of printed-circuit board manufacture, i.e., only when the pattern of conductive paths is produced, which corresponds to the desired circuit diagram. Since this is individually done for most types of printed-circuit boards, the numbers of identical boards having the same arrangement of holes are comparatively small; manufacture is thus rendered expensive.
In the processes which are known in the art, the production of holes and the metallization of hole walls represents a critical process stage, which requires a number of preliminary preparation and cleaning steps and must be carried out with particular care to obtain a uniform, uninterrupted metal layer. In the subtractive and semiadditive methods, this is the only stage in which metal must be deposited in an electroless process.
It is known to catalyze the insulating base material throughout, i.e., to incorporate palladium nuclei in its entire mass. As a result, it is possible to place the connection holes in any position and to provide these holes with a conductive metal coating by electroless deposition. By this procedure, the critical step of seeding with metal nuclei and anchoring the nuclei is obviated at the time of application; however, drilling and electroless metallization must still be carried out in this stage of the process. In addition, a substantially higher amount of noble-metal compounds is required to include metal nuclei in the entire mass of the base material.
German Offenlegungsschrift No. 2,453,788 discloses a process for manufacturing printed circuits, in which a base material for circuit boards is used, which has a metal core covered with insulating layers on both sides thereof and to each insulating layer a circuit diagram of copper is applied. To produce the through-connections, holes are provided in the laminate in the required positions and are filled with plugs comprising an insulating plastic material. Through these plugs, holes of smaller diameters are then drilled, such that a continuous wall of the insulating plastic remains. These walls are thereafter coated with a conductive metal layer which establishes the connection between the two surfaces, without producing a conductive connection to the metal core.
A similar process is described in German Auslegeschrift No. 2,739,494. In this process, a number of holes in excess of the through-connections which are subsequently required are produced in the base material, for example, in a grid arrangement, and are filled with an insulating material. When manufacturing the printed-circuit boards at a later time, the plugs of insulating material are drilled out again only in the required positions and the bores so produced are metallized. By arranging the holes according to a grid system it is intended to enable a uniform serial production of the base material used in printed-circuit boards for digital electronics, which comprise several integrated circuits.
In all these processes, however, through-plating of bores can only be effected individually, corresponding to the desired circuit diagram, at the time of manufacturing the printed circuit. This means that the critical stage of metallizing the hole walls, which comprises many steps of preparation and metal deposition, is invariably subject to the technological disadvantages of individual or small-lot production at the processors of the base material.